A conventional digital amplification device designed in consideration of reduction of coil distortion of a low-pass filter is disclosed in Japanese Laid-open Patent No. 2000-307359.
FIG. 6 is a circuit block diagram of the conventional digital amplification device. ΔΣ modulator (ΔΣ in FIG. 6) 101 converts analog input signal 105 into a pulse density modulation signal. Constant-voltage switch (SW in FIG. 6) 102 is formed of a switch element having a small on-resistance such as a metal-oxide semiconductor field-effect transistor (MOSFET), and power-amplifies an output signal of ΔΣ modulator 101. Low-pass filter (LPF in FIG. 6) 103 is formed of a coil and a capacitor. Cut-off frequency of low-pass filter 103 is set lower than the switching frequency of constant-voltage switch 102. The switching component included in an output signal of constant-voltage switch 102 is attenuated, and a power output of constant-voltage switch 102 is smoothed and demodulated to an analog output signal. Feedback circuit (β in FIG. 6) 104 feeds back an output of low-pass filter 103 to an input of ΔΣ modulator 101. In this configuration, a distortion occurring in low-pass filter 103 is detected based on the output voltage, and is fed back as a negative phase component to the input part to improve the distortion factor.
Distortion of a coil used in the low-pass filter also causes a problem about sound quality. The digital amplification device requires a low-pass filter for integrating pulse signals, but especially coil distortion at a high magnitude output is a large factor of degradation of the sound quality. This distortion is caused by a nonlinear characteristic of a B-H curve of a magnetic material used for the coil, and is current distortion generated by current. The larger the current is, the greater the effect of the nonlinear characteristic is.
Especially, a car audio equipment is often operated by a 12V system as a power source for an automobile, so that a high power output must be obtained by low power source voltage. For this purpose, a method of decreasing impedance of a speaker as a load is used. When the impedance of the load is decreased, however, current of the load increases and hence the distortion of the coil of the low-pass filter in the digital amplification device further increases to degrade the sound quality.
For decreasing the distortion factor of low-pass filter 103 in the digital amplification device, a means for detecting output current is useful. When only a resistor works as a load, the output voltage is proportional to the output current, and therefore the distortion of the coil can be accurately reduced by detecting the output voltage. When a nonlinear load such as a speaker is used, however, the output current can largely distorts even if the output voltage does not distort. It is difficult to correct this distortion by detecting the output voltage.
For increasing the output efficiency, an on-resistance of a switching transistor and a resistance component of an output circuit must be minimized. Here, the switching transistor constitutes the output part of a digital amplifying stage, and the resistance component is, for example, a resistance component of the coil in low-pass filter 3. Therefore, it is not preferable to dispose a resistor or the like in an output circuit in series.